The present invention relates to a gallium oxide (Ga2O3)-zinc oxide (ZnO) series sputtering target (GZO series target) for obtaining a transparent conductive film capable of maintaining a favorable optical transmission factor and conductivity, as well as to a method of forming a transparent conductive film using such a target and a transparent conductive film formed thereby.
Conventionally, as a transparent conductive film, an ITO film (i.e., film in which tin is doped in indium oxide) is superior in transparency and conductivity, and is widely used as a transparent electrode (film) of display devices such as a liquid crystal display and an electro luminescence display, as well as in solar batteries. Nevertheless, since indium, which is the primary component of ITO, is expensive, there is a problem in that the manufacturing cost will increase.
Due to the foregoing reasons, the usage of a GZO film as a substitute for the ITO film is being proposed. This GZO film is advantageous since it is a zinc oxide series film having gallium oxide (Ga2O3)-zinc oxide (ZnO) as its primary component, and is inexpensive. The GZO film is known to encounter a phenomenon of increased conductivity due to the oxygen defect of ZnO, which is the primary component thereof, and the increased use of such GZO film can be realized if the film characteristics such as conductivity and optical transparency come near the ITO film.
As a method of forming the GZO film, the sputtering method is primarily used, and, in particular, direct current (DC) sputtering, radio frequency (RF) sputtering or magnetron sputtering is used from the perspective of operability and film stability.
Formation of a film based on the sputtering method is conducted by physically colliding a positive ion such as an Ar ion to a target disposed on a negative electrode, using such collision energy to discharge a material for configuring a target, and laminating a film having roughly the same composition as the target material on a substrate on the opposite positive electrode.
The coating based on the foregoing sputtering method is characterized in being able to form a thin film of angstrom units to a thick film of several ten μm with a stable deposition speed by adjusting the processing time and power supply.
Several proposals have been made regarding a sintered body sputtering target for forming this kind of GZO film, and a transparent conductive film formed thereby.
For example, Patent Document 1 proposes a zinc oxide series sintered body target in which an abnormal electrical discharge does not occur in a part thereof and which is able to form a stable thin film. In Patent Document 1, a Ga2O3—ZnO target sintered body is used as a part of the target material, and zinc oxide in which titanium oxide, germanium oxide, aluminum oxide, magnesium oxide, indium oxide, and tin oxide is selectively added at 1 to 5 weight % is used as the primary component.
Patent Document 2 proposes a GZO sintered body sputtering target without any abnormal electrical discharge and which is able to form a stable thin film. In Patent Document 2, technology is proposed for pulverizing the powders of zinc oxide and gallium oxide to a grain size of 1 μm or less, adjusting the sintering temperature at 1300 to 1550° C., and improving the density by performing sintering while introducing oxygen.
Patent Document 3 proposes a GZO sintered body sputtering target with a high transmission factor and a low resistance value and in which the occurrence of an abnormal electrical discharge is rare over a long period of time. In Patent Document 3, proposed is a ZnO series sintered body in which Ga is added at 3 to 7 atomic %, and a third element selected from Al, B, In, Ge, Si, Sn, and Ti is added at 0.3 to 3 atomic %.
Patent Document 4 proposes technology of performing sputtering in an atmosphere of hydrogen gas and inert gas in order to prevent changes in the electrical characteristics and optical characteristics due to zinc oxide reacting with moisture.
Generally speaking, a major problem in forming a GZO film is that minute protrusions called nodules occur in the erosion portion of the target surface due to sputtering, coarse grains (particles) float in the sputtering chamber caused by the abnormal electrical discharge and splash due to such nodules, and such particles adhere to the formed film and cause deterioration in quality. Further, the foregoing abnormal electrical discharge will cause an unstable plasma discharge, and there is a problem in that stable deposition cannot be realized.
Therefore, upon forming a conductive film on a substrate, it is necessary to periodically remove the nodules occurring on the sputtering target, and there is a problem in that this significantly deteriorates the productivity. Thus, a target which does not generate nodules or an abnormal electrical discharge phenomenon is in demand.
In particular, recently, the tendency is that displays are being enlarged, and deposition on a large area is required. Thus, a target capable of stable deposition is in particular being demanded.
In the foregoing patent documents, the problem of abnormal electrical discharge is being pointed out, and, as measures for reducing such abnormal electrical discharge, in Patent Document 1 described above, titanium oxide, germanium oxide, aluminum oxide, magnesium oxide, indium oxide, and tin oxide is selectively added at 1 to 5 weight %. In Patent Document 3, a third element selected from Al, B, In, Ge, Si, Sn, and Ti is added at 0.3 to 3 atomic %.
Each of the foregoing documents attempts to prevent the abnormal electrical discharge by increasing the density of the sintered body and reducing the holes in the sintered body. Nevertheless, even with the use of these additive materials, there is a problem in that the sintered density cannot be sufficiently increased, and the bulk (volume) resistance value remains high.
Further, although it is possible to improve the manufacture process of the target, a complex manufacture process will cause increased costs. In addition, when attempting to increase the density by improving the sintering method or device, there is a problem in that the equipment must be enlarged, and this cannot be considered to be an industrially efficient method.
Comprehensively, by adding trace elements; that is, by changing the component composition of the GZO sintered body, it is possible to improve the target density, prevent the formation of nodules, and inhibit the generation of an abnormal electrical discharge phenomenon and particles, and this would be a simple and effective method. Nevertheless, change in the component composition will aggravate the bulk resistance value of the target, and, since the sintered density will not necessarily be improved, there is a problem in that the examples described in the foregoing Patent Documents are insufficient as measures in overcoming the conventional problems.
As technology where the component composition is approximate, there is an optical disk protective film and a sputtering target for forming such a protective film (refer to Patent Document 5). Nevertheless, this technology is for use in an optical disk protective film, and one or two among ZnO, In2O3 or SnO2 are used as the primary component, and Al2O3 or Ga2O3 or ZrO2 is additionally included. For application as an optical disk protective film, the optimal range of adding Ga2O3 is described as being 0.1 to 20 wt %, and the optimal range of adding ZrO2 is described as being 0.01 to 5 wt %.
Here, needless to say, the objective is to obtain an optical disk protective film, and it does not possess the function as a conductive film. This is a matter of course since Patent Document 5 is for use as an optical disk protective film, but the technology disclosed in Patent Document 5 has no recognition of obtaining the availability as a transparent conductive film or the conductivity in relation thereto.    [Patent Document 1] Japanese Patent Laid-Open Publication No. H10-306367    [Patent Document 2] Japanese Patent Laid-Open Publication No. H10-297964    [Patent Document 3] Japanese Patent Laid-Open Publication No. H11-256320    [Patent Document 4] Japanese Patent Laid-Open Publication No. 2002-363732    [Patent Document 5] Japanese Patent Laid-Open Publication No. 2000-195101